Various apparatuses and methods have been proposed in prior art for measuring refractive index of a sample such as gases, liquids, etc. As one representative apparatuse for measuring refractive index, an Abbe's refractometer has been known which measures refractive index by using a prism for studying critical angle of total reflection of a sample.
A refractive index measurement apparatus using interference of light has also been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2002-168780).
Further, a refraction index measurement method using diffraction of light has also been proposed. As one such measurement method, Japanese Unexamined Patent Application Publication No. 2006-170727 discloses an evaluation method for evaluating a polarized beam splitter to measure refractive index modulation quantity of a polarized beam splitter as a diffraction device in which a composition comprising a non-polymerizable liquid crystal, a polymerizable monomer and a photo-polymerization initiator is exposed to light by a two-beam interference exposure method in order to form a periodic structure consisting of a region exhibiting optical anisotropy and a region exhibiting optical isotropy.
In this method, while temperature of the polarized beam splitter is controlled, the polarized beam splitter is irradiated with a laser beam to measure diffraction efficiency, and from the relationship between refractive index modulation quantity and the diffraction efficiency, and by using temperature characteristics of the diffraction efficiency, the refraction index modulation quantity corresponding to the diffraction efficiency is identified.
Further, Japanese Unexamined Patent Application Publication No. 2007-292509 discloses a refractive index measurement method in which refraction index of a sample covering a diffraction device is determined by measuring zero order diffraction efficiency of a diffraction device, and identifying the refractive index as one that best matches the measured zero order diffraction efficiency and the theoretical diffraction efficiency calculated by using the grating shape of the diffraction device.